1. Field of the Invention
This invention relates to a relay system for covering silent zones in a mobile communications system, and in particular to a relay system capable of providing radio communication of desired communication quality throughout a complicated-shaped silent zone, repeating various kinds of signals with a simple configuration, and facilitating the system management.
2. Description of the Related Art
A public vehicular communications system such as a mobile and portable telephone system, a personal handyphone system (PHS) or a radio paging system (hereinbelow, generically called a xe2x80x9cmobile communications systemxe2x80x9d) is equipped with vehicular communication base stations (hereinbelow, simply called xe2x80x9cbase stationsxe2x80x9d) connected by wire to an upper network such as a public telecommunication network (PSTN or ISDN), in which the base stations realize radio communication with mobile stations such as portable telephones.
In case of the mobile and portable telephone system, each base station has an area communicable by radio (coverage area) that ranges from several kilometers to ten and several kilometers. To secure a wider service area while effectively using the frequencies, such a system that coverage areas are arranged like cells, called a cellular system, has been adopted.
In a mobile communications system using such a cellular system, it would be possible to provide good radio communication with mobile radio terminals in outdoor environments such as ones on the ground, but difficult to realize radio communication with terminals inside a tunnel, an underground shopping center, a basement and aboveground stories in a large-scale building because of the difficulty of delivering radio waves.
Such an area as to make radio communication impossible is called a xe2x80x9csilent zone,xe2x80x9d and commonly appears not only in the mobile communications but also any other communications using radio waves, such as radio communications for business use including police radio, fire fighting radio and train radio, television broadcasting and radio broadcasting.
To enable radio communication in the silent zone, an apparatus, called a repeater-amplifier, is typically used as auxiliary means.
The apparatus is accompanied with a first antenna installed at a point to which a base station (or a broadcasting station) can transmit radio waves for proper communication, and a second antenna installed in the silent zone. In such a configuration, an incoming radio-frequency signal to the first antenna is amplified and transmitted through a cable to the second antenna in the silent zone. The amplified radio-frequency signal is then radiated from or broadcast through the second antenna.
A down link from the base station to a mobile station can thus be secured. An up link from a mobile station to the base station can also be allowed by amplifying a radio-frequency signal from the mobile station, transmitting it to the first antenna through the cable and radiating the amplified radio-frequency signal from the first antenna for transmission to the base station. It is therefore possible to realize radio communication in the silent zone.
So far, two methods of installing the second antenna have been used: one using a leakage coaxial system and the other using an optical fiber system. But in using either of the methods without any consideration, especially for mobile communications such as portable telephones, the installation of the second antenna can merely expand the coverage area of the base station at which a relay is received from the silent zone. This increases the amount of communication to be processed by the base station, and may cause the base station to exceed its capacity when used in an area such as a metropolitan area in which demand for communication is so great that incoming and outgoing lines from portable stations in the coverage area could not be processed.
Since an upper wired network of high quality such as an ISDN network has recently been popularized, installation of a radio modulator-demodulator directly connected by wire to the upper network such as ISDN is considered nowadays.
Such a radio modulator-demodulator modulates a radio-frequency signal with an input signal from the upper network and distributes or allots the radio-frequency signal to an antenna in the silent zone (the above-mentioned second antenna), while it receives a radio-frequency signal from a mobile station in the silent zone through the antenna and demodulates it for output to the upper network.
Referring now to FIGS. 9 and 10, such conventional relay systems will be described. FIG. 9 is a diagram for explaining the concept of a conventional relay system using a leakage coaxial cable, and FIG. 10 is a diagram for explaining the concept of another conventional relay system using optical fiber.
The relay system using a leakage coaxial cable shown in FIG. 9 is constituted of a public telecommunication network 1 such as PSTN or ISDN, a radio modulator 2 connected by wire to the public telecommunication network 1, a coaxial cable 3, an amplifier 4, a leakage coaxial cable 5 and auxiliary amplifiers 6.
In FIGS. 9 and 10, only the down link is shown for simplification.
Hereinbelow, description is specifically made with respect to each portion.
The radio modulator 2 modulates a carrier with an incoming signal from the public telecommunication network 1 through a cable to obtain a radio-frequency signal and outputs the radio-frequency signal through the coaxial cable 3.
The amplifier 4 amplifies the radio-frequency signal taken in through the coaxial cable 3 and outputs the amplified signal to the leakage coaxial cable 5.
Parts of the leakage coaxial cable 5 are disposed linearly across the silent zone so as to transmit the radio-frequency signal from one end to the other while gradually leaking it to the outside. Thus the leakage coaxial cable 5 serves as an antenna.
Each auxiliary amplifier 6 is inserted between two sections or parts of the leakage coaxial cable 5 to amplify the radio-frequency signal passing through the leakage coaxial cable 5 so as to extend the extension distance of the leakage coaxial cable 5.
Next, operation of the relay system using the leakage coaxial cable of FIG. 9 will be described.
The radio modulator 2 modulates a radio-frequency signal with the incoming signal from the public telecommunication network 1 for output to the amplifier 4 through the coaxial cable 3.
The amplifier 4 amplifies the radio-frequency signal for output to the leakage coaxial cable 5.
The leakage coaxial cable 5, like an antenna, transmits the radio-frequency signal from the amplifier 4 while leaking it to the outside. The auxiliary amplifier 6 amplifies the radio-frequency signal attenuated due to a leak from the preceding part of the leakage coaxial cable 5 and outputs it to the subsequent part of the leakage coaxial cable 5. The subsequent part of the leakage coaxial cable 5 in turn leaks the amplified radio-frequency signal. Thus, the radio-frequency signal is imparted through the public telecommunication network 1 to a mobile station existing in the silent zone.
The relay system using optical fiber shown in FIG. 10 is constituted of a public telecommunication network 1, optical fiber 7, a fixed central station 10, an optical star coupler 20 and a plurality of fixed relay stations 30.
The fixed central station 10 includes, in addition to a radio modulator 2 and an amplifier 4 similar to those in the relay system of FIG. 9, an electro-optic transducer (E/O) transducer 11 for transducing the radio-frequency signal amplified by the amplifier 4 into an optical signal. Each of the fixed relay stations 30 includes, in addition to an amplifier 34 similar to the amplifier 4 in the relay system of FIG. 9, an opto-electric (O/E) transducer 31 for transducing the optical signal into a radio-frequency signal and an antenna 32 from which the signal amplified by the amplifier 34 is radiated.
Hereinbelow, description is specifically made with respect to each portion.
The radio modulator 2 of the fixed central station 10 converts the signal input from the public telecommunication network 1 into a radio-frequency signal for output to the amplifier 4.
The amplifier 4 amplifies the radio-frequency signal from the radio modulator 2 for output to the E/O transducer 11.
The E/O transducer 11 transduces the amplified radio-frequency signal into an optical signal for output to the optical star coupler 20.
Thus, the fixed central station 10 converts the signal input from the public telecommunication network 1 into a radio-frequency signal, amplified the radio-frequency signal, modulates the carrier of an optical signal with the amplified radio-frequency signal, transduces the signal into the optical signal for output to the optical star coupler 20.
The optical star coupler 20 distributes the input optical signal to the plurality of fixed relay stations 30 through optical fiber lines 7.
A number of fixed relay stations 30 are arranged in a silent zone, each of which transduces the input optical signal into a radio-frequency signal through the O/E transducer 31 and amplifies it at the amplifier 34 for radiation to the outside through the antenna 32.
In general, one antenna has a limited area for radio communication of desired quality, depending on the gain and directivity of the antenna 32 and its radio-frequency output. Therefore, the number of fixed relay stations 30 varies according to the dimensions of the silent zone so that radio communication of desired quality can be provided throughout the silent zone.
Next, operation of the conventional relay system using optical fiber shown in FIG. 10 will be described.
At first, the fixed central station 10 modulates a carrier with the incoming signal from the public telecommunication network 1 to obtain a radio-frequency signal, amplifies the radio-frequency signal and transduces amplified one into an optical signal for output to the optical star coupler 20.
The optical star coupler 20 distributes the optical signal and outputs it to the plurality of fixed relay stations 30 through respective optical fiber lines 7. Each of the fixed relay stations 30 transduces the input optical signal into the radio-frequency signal, amplifies and radiates it from the antenna 32. The signal input from the public telecommunication network 1 can thus be transmitted to the silent zone.
The conventional relay system using the leakage coaxial cable is adapted to a linear-shaped silent zone such as a tunnel. However, since the leakage coaxial cable leaks a constant amount of high-frequency electric power per unit length to the outside in the form of electromagnetic waves, the loss of radio-frequency signals increases in a range of high-frequency bands such as 1.5 GHz and 1.9 GHz recently allocated for each equipment such as a portable telephone or PHS, in addition to the loss due to leakage. It is therefore difficult to maintain desired communication quality in the silent zone.
Further, a coming mobile communications system, called an FPLMTS, is expected to use a band of 2 GHz as worldwide common allocation. When transmitting signals in such a high-frequency band, a large number of auxiliary amplifiers must be inserted in order to compensate the loss of radio-frequency signals from the leakage coaxial cable. In this case, the auxiliary amplifiers would be inserted at intervals of about 100 meters. On the other hand, the number of connectable auxiliary amplifiers and parts of the leakage coaxial cable is limited depending on the wave quality such as required distortion and noise, and the degree of distortion and the level of noise generated in the amplifier and the auxiliary amplifier. This makes it difficult to lay an extended leakage coaxial cable with many auxiliary amplifiers, and if the silent zone is wide, it may be found impossible to cover the entire silent zone.
Furthermore, the diameter of the coaxial cable for wire-connecting the radio modulator and the amplifier must be large enough to cope with such a high frequency, and this also raises the difficulty in construction work.
In contrast, the conventional relay system using optical fiber is easy to construct because optical fiber has a small outside diameter of about 125 xcexcm and is flexible and light. Optical fiber also has a low-loss characteristic, the attenuation of which is 0.5 dB or less per one-kilometer transmission distance, and a sufficient transmission bandwidth of several THz per one-kilometer transmission distance. These characteristics of optical fiber can cope with the high frequencies and is enough to keep an adequate transmission distance, but since waves are radiated from the antenna in the form of a spot, it would also be difficult to provide radio communication of desired communication quality throughout a silent zone spreading in a line such as a tunnel or underpass.
To compensate mutual disadvantages of the relay system using the leakage coaxial cable and the relay system using the optical fiber, the third approach is considered in which the system is based on the relay system using optical fiber except for the use of a leakage coaxial cable instead of antennas.
For example, such an invention is disclosed in Japanese application laid-open publication No. 9-130322, entitled xe2x80x9cRelay Amplification System for Vehicular Communication,xe2x80x9d but this application does not take into account a relay of various kinds of signals including ones for communication through a public telecommunication network other than radio communications. In this system, if such various kinds of signals need repeating, each individual relay station must be provided for each type of signal, and this also makes it difficult to construct the system.
It is an object of the present invention to provide a relay system capable of providing radio communication of desired communication quality throughout a complicated-shaped silent zone, repeating various kinds of signals with a simple configuration, and facilitating the system management.
According to the present invention, there is provided a relay system comprising a fixed central station, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal; and a plurality of fixed relay stations, distributed in a silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, wherein said fixed central station includes a radio modulator for modulating a specified carrier frequency with a signal input from a public telecommunication network to obtain the radio-frequency signal, whereby any mobile station can receive radio signals of desired communication quality from the public telecommunication network.
In the second aspect of the present invention, there is provided a relay system comprising a fixed central station, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal, and transduces into a radio-frequency signal an optical signal input from a silent zone through optical fiber; and a plurality of fixed relay stations, distributed in the silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, and transduces into an optical signal a radio-frequency signal received through the leakage coaxial cable from a mobile station existing in the silent zone for output to said fixed central station, wherein said fixed central station includes a radio modulator for modulating a specified carrier frequency with a signal input from a public telecommunication network to obtain the radio-frequency signal, and a radio demodulator for demodulating the radio-frequency signal transduced from the optical signal from said fixed relay station for output to the public telecommunication network, whereby any down-link signal from the public telecommunication network can be repeated throughout the silent zone, while any up-link signal radiated from mobile stations existing in the silent zone can be repeated to the public telecommunication network.
In the third aspect of the present invention, there is provided a relay system comprising a fixed central station, installed outside of silent zones, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal, and transduces into a radio-frequency signal an optical signal input from a silent zone through optical fiber; and a plurality of fixed relay stations, distributed in the silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, and transduces into an optical signal a radio-frequency signal received through the leakage coaxial cable from a mobile station existing in the silent zone for output to said fixed central station, wherein said fixed central station includes an antenna, a radio part and a wired part; said radio part outputs an incoming signal from said antenna as a radio-frequency signal and takes in a radio-frequency signal to radiate the input radio-frequency signal from said antenna to the outside; and said wired part includes plural radio modulators for modulating individually specified carrier frequencies with various kinds of signals input from the outside through cables to obtain radio-frequency signals, a synthesizer for combining the radio-frequency signal from said radio part with the radio-frequency signals from said plural radio modulators to output a combined radio-frequency signal, an electro-optic transducer for transducing the combined radio-frequency signal into an optical signal for output to said fixed relay stations, an opto-electric transducer for transducing into radio-frequency signals the optical signals input from said fixed relay stations, a first distributor for distributing the radio-frequency signals input from said opto-electric transducer into a radio-frequency signal to be radiated from said antenna of said radio part and radio-frequency signals to be output to the outside through the cables, a second distributor for distributing the radio-frequency signals input from said first distributor for output to the outside through the cables, and plural radio demodulators for demodulating various kinds of signals from the radio-frequency signals respectively input from said second distributor, whereby various kinds of signals can be repeated with a simple configuration in such a way that any down-link signal from a public telecommunication network is repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone is repeated to the public telecommunication network.
In the fourth aspect of the present invention, there is provided a relay system comprising a fixed central station, installed outside of silent zones, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal, and transduces into a radio-frequency signal an optical signal input from a silent zone through optical fiber; and a plurality of fixed relay stations, distributed in the silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, and transduces into an optical signal a radio-frequency signal received through the leakage coaxial cable from a mobile station existing in the silent zone for output to said fixed central station, wherein each of said fixed relay stations includes an opto-electric transducer for transducing the optical signal from said fixed central station into a radio-frequency signal, a distributor for distributing the radio-frequency signal to two branches, a demodulator for demodulating a control signal from the radio-frequency signal distributed to one branch for control of the fixed relay station, a control unit for controlling each part of the fixed relay station in accordance with the control signal, a multiplexer for outputting the radio-frequency signal distributed to the other branch to the leakage coaxial cable and outputting an incoming radio-frequency signal from the leakage coaxial cable, and an electro-optic transducer for transducing the radio-frequency signal from said multiplexer into an optical signal for output to said fixed central station, whereby even when the silent zone is of linear structure such as a tunnel or underpass, any down-link signal from a public telecommunication network can be repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone can be repeated to the public telecommunication network, while remotely controlling system setting to said fixed relay stations.
In the fifth aspect of the present invention, there is provided a relay system comprising a fixed central station, installed outside of silent zones, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal, and transduces into a radio-frequency signal an optical signal input from a silent zone through optical fiber; and a plurality of fixed relay stations, distributed in the silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, and transduces into an optical signal a radio-frequency signal received through the leakage coaxial cable from a mobile station existing in the silent zone for output to said fixed central station, wherein each of said fixed relay stations includes an opto-electric transducer for transducing the optical signal from said fixed central station into a radio-frequency signal, a multiplexer for outputting the radio-frequency signal to the leakage coaxial cable and outputting an incoming radio-frequency signal from the leakage coaxial cable, a control unit for monitoring the operation status of each part of the fixed relay station to output a supervisory signal, a modulator for modulating the supervisory signal into a radio-frequency signal, a synthesizer for combining the radio-frequency signal modulated by said modulator and the radio-frequency signal input from said multiplexer, and an electro-optic transducer for transducing the radio-frequency signal from said synthesizer into an optical signal for output to said fixed central station, whereby even when the silent zone is of linear structure such as a tunnel or underpass, any down-link signal from a public telecommunication network can be repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone can be repeated to the public telecommunication network, while remotely monitoring the operation status of said fixed relay stations.
In the sixth aspect of the present invention, there is provided a relay system comprising a fixed central station, installed outside of silent zones, which transduces a radio-frequency signal into an optical signal to output the optical signal for repeating the radio-frequency signal, and transduces into a radio-frequency signal an optical signal input from a silent zone through optical fiber; and a plurality of fixed relay stations, distributed in the silent zone, each of which transduces into a radio-frequency signal the optical signal input from said fixed central station through optical fiber for radiation into the silent zone through a leakage coaxial cable laid across the silent zone, and transduces into an optical signal a radio-frequency signal received through the leakage coaxial cable from a mobile station existing in the silent zone for output to said fixed central station, wherein each of said fixed relay stations includes an opto-electric transducer for transducing the optical signal from said fixed central station into a radio-frequency signal, a distributor for distributing the radio-frequency signal to two branches, a demodulator for demodulating a control signal from the radio-frequency signal distributed to one branch for control of the fixed relay station, a control unit for controlling each part of the fixed relay station in accordance with the control signal and monitoring the operation status of each part of the fixed relay station to output a supervisory signal, a multiplexer for outputting the radio-frequency signal distributed to the other branch to the leakage coaxial cable and outputting an incoming radio-frequency signal from the leakage coaxial cable, a modulator for modulating the supervisory signal into a radio-frequency signal, a synthesizer for combining the radio-frequency signal modulated by said modulator and the radio-frequency signal input from said multiplexer, and an electro-optic transducer for transducing the radio-frequency signal from said synthesizer into an optical signal for output to said fixed central station, whereby even when the silent zone is of linear structure such as a tunnel or underpass, any down-link signal from a public telecommunication network can be repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone can be repeated to the public telecommunication network, while remotely controlling recovery from abnormality in a fixed relay station and system setting to the fixed relay station.
In the seventh aspect of the present invention, there is provided a fixed central station installed outside of silent zones, comprising an antenna, a radio part, a wired part and a control unit, which transduces a radio-frequency signal into an optical signal for repeating the radio-frequency signal to output the optical signal through optical fiber to fixed relay stations installed in a silent zone, and transduces into a radio-frequency signal an optical signal input from each fixed relay station through optical fiber, wherein said radio part outputs an incoming signal from said antenna as a radio-frequency signal and takes in a radio-frequency signal to radiate the input radio-frequency signal from said antenna to the outside; said wired part includes plural radio modulators for modulating individually specified carrier frequencies with various kinds of signals input from the outside or said control unit through cables to obtain radio-frequency signals, a synthesizer for combining the radio-frequency signal from said radio part with the radio-frequency signals from said plural radio modulators to output a combined radio-frequency signal, an electro-optic transducer for transducing the combined radio-frequency signal into an optical signal for output to said fixed relay stations, an opto-electric transducer for transducing into radio-frequency signals the optical signals input from said fixed relay stations, a first distributor for distributing the radio-frequency signals input from said opto-electric transducer into a radio-frequency signal to be radiated from said antenna of said radio part and radio-frequency signals to be output to the outside through cables, a second distributor for distributing the radio-frequency signals input from said first distributor for output to the outside through the cables, and plural radio demodulators for demodulating various kinds of signals from the radio-frequency signals respectively input from said second distributor; and said control unit, connected to one of said radio modulators and one of said radio demodulators, recognizes the operation status of each fixed relay station based on a supervisory signal input from the fixed relay station through the radio demodulator to output a control signal to the radio modulator for control of the fixed relay station in accordance with the operation status, whereby even when the silent zone is of linear structure such as a tunnel or underpass, various kinds of signals can be repeated with a simple configuration using a pair of optical fiber cables in such a way that any down-link signal from a public telecommunication network is repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone is repeated to the public telecommunication network, while remotely controlling recovery from abnormality in a fixed relay station and system setting to the fixed relay station by monitoring the operation status of each fixed relay station at said fixed central station.
In the eighth aspect of the present invention, said control unit of said fixed central station outputs a signal indicative of the operation status of each fixed relay station to an upper monitor station located outside, and outputs a control signal to the radio modulator for control of the fixed relay station in accordance with an instruction signal from the upper monitor station, thereby remotely controlling recovery from abnormality in a fixed relay station and system setting to the fixed relay station while monitoring the operation status of each fixed relay station at the upper monitor station.
In the ninth aspect of the present invention, said control unit of said fixed central station outputs to the modulator such a control signal as to normalize a fixed relay station based on the supervisory signal from the fixed relay station when said fixed central station recognizes occurrence of abnormality in the fixed relay station, thereby normalizing a fixed relay station immediately when abnormality occurs in the fixed relay station while monitoring the operation status of each fixed relay station at said central station.
In the tenth aspect of the present invention, said relay system according to the sixth aspect of the present invention comprises said fixed central station according to the seventh aspect of the present invention, whereby even when the silent zone is of linear structure such as a tunnel or underpass, various kinds of signals can be repeated with a simple configuration using a pair of optical fiber cables in such a way that any down-link signal from a public telecommunication network is repeated throughout the silent zone and any up-link signal radiated from mobile stations existing in the silent zone is repeated to the public telecommunication network, while remotely controlling recovery from abnormality in a fixed relay station and system setting to the fixed relay station by monitoring the operation status of each fixed relay station at said fixed central station.
In the eleventh aspect of the present invention, said relay system according to the sixth aspect of the present invention comprises said fixed central station according to the eighth aspect of the present invention, thereby remotely controlling recovery from abnormality in a fixed relay station and system setting to the fixed relay station while monitoring the operation status of each fixed relay station at the upper monitor station.
In the twelfth aspect of the present invention, said relay system according to the sixth aspect of the present invention comprises said fixed central station according to the ninth aspect of the present invention, thereby normalizing a fixed relay station immediately when abnormality occurs in the fixed relay station while monitoring the operation status of each fixed relay station at said central station.